1. Field of the Invention
The present invention relates to a circuit apparatus for driving a magnetic head to be used for magnetic modulation in a magnetic modulation type optomagnetic recording apparatus.
2. Related Background Art
Conventionally, optical modulation systems, magnetic-field modulation systems and the like are known as a recording system of an optomagnetic recording apparatus. Among them, the magnetic-field modulation system is particularly advantageous in its recording speed, for example, since new data can be directly overwritten onto old data in this modulation system. FIG. 1 illustrates a schematic structure of such a conventional magnetic-field modulation system. In FIG. 1, reference numeral 1 designates an optomagnetic disc to be used as an information recording medium, and reference numeral 2 designates an optomagnetic recording layer formed in the disc 1. In this structure, a magnetic head 6 is disposed above the optomagnetic disc 1, and an optical head 4 is disposed below the disc 1, opposite to the magnetic head 6. The optical head 4 causes a laser beam 7 from a semiconductor laser, which is arranged in the optical head 4 as a light source, to be applied onto the recording layer 2 as a minute light spot so that the temperature of a recording portion is raised to a value higher than the Curie temperature. On the other hand, the magnetic head 6 generates a bias magnetic field modulated in accordance with information to be recorded by driving a driver circuit 3, and applies this bias magnetic field to a temperature-raised portion of the recording layer 2. As a result, the orientation of magnetization in the temperature-raised portion of the recording layer 2 is changed to a direction of the bias magnetic field, and the information is recorded in the recording layer 2.
As a driving apparatus for a magnetic head of a magnetic-field modulation system, an apparatus as disclosed in, for example, Japanese Patent Laid-open No. 63-94406 is known. FIG. 2 shows a circuit diagram of this driving apparatus. In FIG. 2, reference symbol L.sub.H designates a chief coil for generating a bias magnetic field, reference symbol L.sub.1 and L.sub.2 respectively designate auxiliary coils for speedily changing over a magnetic field, reference symbol SW.sub.1 and SW.sub.2 respectively designate switch elements for changing the direction of current in the chief coil L.sub.H, and reference symbol R.sub.1 and R.sub.2 respectively designate resistors for limiting current. Values of inductances of the auxiliary coils L.sub.1 and L.sub.2 are set to values larger than the inductance of the chief coil L.sub.H. In this driving apparatus, the switch elements SW.sub.1 and SW.sub.2 are controlled to be alternately turned on or off (closed or open), and the polarity of the generated magnetic field is switched in accordance with information to be recorded by switching the direction of current in the chief coil L.sub.H. In more detail, current paths CH.sub.1 and CH.sub.4 are closed or established while current paths CH.sub.2 and CH.sub.3 designated by dashed lines are open or nonexistent, when the switch element SW.sub.1 is closed and the switch element SW.sub.2 is opened. Then, since current is supplied to the chief coil L.sub.H due to the establishment of the current path CH.sub.1, a magnetic field in accordance with the direction of the current is generated by the chief coil L.sub.H.
On the other hand, when the switch element SW.sub.1 is opened and the switch element SW.sub.2 is closed, the current paths CH.sub.2 and CH.sub.3 are closed or established while the current paths CH.sub.1 and CH.sub.4 are open or nonexistent. As a result, current opposite in direction to the above-mentioned current (i.e., a reverse current) flows through the chief coil L.sub.H due to the establishment of the current path CH.sub.2, and a magnetic field whose polarity is reversed is generated. Here, since the inductances of the auxiliary coils L.sub.1 and L.sub.2 are larger than the inductance of the chief coil L.sub.H, the flowing currents are respectively maintained at approximately constant values, while the current path is changed from CH.sub.1 to CH.sub.3, and from CH.sub.4 to CH.sub.2 as the switch elements SW.sub.1 and SW.sub.2 are changed over. Therefore, if the on-off time of the switch elements SW.sub.1 and SW.sub.2 is made sufficiently short, the direction of the current flowing through the chief coil L.sub.H can be reversed in a very short time without increasing the voltage of a DC source V. This is effective for preventing mistakes in recording information signals in order to achieve a desired signal recording.
However, when the switch element is, for example, a field effect transistor, a stray capacitance between drain and source appears even if the switch element is actually open. As a result, as shown in FIG. 3, a vibration phenomenon due to such stray capacitance and the inductance component of the chief coil L.sub.H occurs, and the time required for reversing the current of the chief coil L.sub.H is determined by a period of this vibration. Therefore, in order to shorten the reversal time, a switch element having a speedy switching time and a small stray capacitance should be used. Further, the vibration is gradually attenuated as its energy is consumed, but in order to prompt this attenuation, a resistor (not shown) should be used to consume the vibration energy. Thus, current shown in FIG. 3 is supplied to the chief coil L.sub.H by alternate on-off control of the switch elements SW.sub.1 and SW.sub.2, and a magnetic field corresponding to the direction of this current is generated. In FIG. 3, a dashed line indicates a vibration wave shape due to the inductance and the stray capacitance, and a solid line indicates a current wave shape in a case where the vibration is speedily attenuated using the resistor and the like.
In the prior art magnetic head driving apparatus, however, most of the magnetic energy (1/2.multidot.LI.sup.2 ; L is an inductance of the chief coil L.sub.H and I is the current passing through it) stored in the chief coil L.sub.H is consumed in the resistor and so forth in a process of the vibration phenomenon, and at this time energy of 1/2.multidot.LI.sup.2 is momentarily supplied from the auxiliary coil to the chief coil L.sub.H as current in an opposite direction, thereby reversing the current through the chief coil L.sub.H. Therefore, the reduced energy of 1/2.multidot.LI.sup.2 must be supplied to the auxiliary coil from the DC source V by the next current reversal. Generally, when a current supply is performed from a source having a voltage of V to an auxiliary coil having an inductance of L.sub.A, the rate of change .DELTA.i/.DELTA.t of the current relative to time is V/L.sub.A, and hence the energy supply from the DC source having a low voltage to the auxiliary coil having a large inductance takes a considerable time. Therefore, in a case where the current reversal is conducted at a high repetitive frequency, current supplied to the chief coil is reduced since the current reversal is performed in a state in which the energy supply to the auxiliary coil inadequate. Thus, a normal recording of information becomes impossible. Actually, when the voltage of the source V is 5V, the inductance of the chief coil L.sub.H is 1 .mu.H and the current supplied to the chief coil is 0.2A, an upper limit of the driving frequency is about 5 MHz. As this frequency increases, the current decreases and information recording degradation occurs.